Liquid Crystal (LC) displays (LCD) and lenses (LCL) are known in the art. In large majority of cases that use LCs, an electrically variable gradient index (so-called GRIN) optical lens is formed by controlling the relative orientation gradient of LC molecules in space within a Clear Aperture (CA) of the device. Then, this molecular orientation, being sensitive to the electrical field, the gradient (and respectively, the optical power of the LCL) can be changed by changing the electrical stimulus parameters (voltage, frequency or a combination thereof) without any macroscopic mechanical movement or deformation.
A variety of LCL designs have been proposed which control the orientation of the LC molecules in response to a spatially non-uniform electric field, see for example the review of S. Sato, “Applications of Liquid Crystals to Variable-Focusing Lenses”, OPTICAL REVIEW Vol. 6, No. 6 (1999) 471-485. One approach is the use of multiple electrode arrays (such as used in LCDs) to generate a lens-like electric field profile in space. However, the complexity of its manufacturing and of its dynamic control reduces its attractiveness and industrial acceptance.
Another approach 100 was described by S. Sato above which uses a combination of a Hole-Patterned Electrode (HPE) 102 and a Transparent Uniform Electrode (TUE) 104 (on bottom substrate 105), FIG. 1A (Hereafter, the LC alignment layers and other standard elements of LC cell will be omitted to simplify the drawings. Also, schematic cross-sectional views of various designs and embodiments will be mainly presented for the same reasons). The application of an electrical potential difference 106 across the two electrodes 102 & 104 will generate a spatially non-uniform electric field (between points ACB, as schematically illustrated in FIG. 1B). Thus, if the electrical potential distribution (and the corresponding electric field) have the appropriate spatial profile, then the corresponding reorientation of LC molecules 108 and the refractive index modulation within the Clear Aperture (CA) 110 of the device 100 may have the desired spherical (or aspherical, see hereafter) form thus enabling a good quality lens 100. The Optical Power (OP) of such a lens (measured in Diopters) may be expressed as:OP=2LΔn/r2,(in the case of a spherical waveform)
where L is the thickness of the LC layer 112, Δn is the difference of refractive index in the center (around the point C) and at the periphery (around points A and B) of the lens 100 and r is the radius of the CA 110. Note that the focal distance F (measured in meters) is the inverse of OP, F=1/OP. This design being much simpler to manufacture, still has some important drawbacks. Namely, the distance (defined by the LC thickness L and the thickness of the top substrate H) between the HPE 102 and TUE 104 must be relatively large to ensure smooth spatial profile (150) of the electric field inside the LC layer 112. This electrode separation L+H necessarily increases the voltages (several tens of volts) required for the control of the LCL 100.
In an article published by A. F. Naumov et al., entitled “Liquid-Crystal Adaptive Lenses with Modal Control”, OPTICS LETTERS/Vol. 23, No. 13/Jul. 1, 1998, an LCL 200 configuration was proposed (shown in FIG. 2), which uses an HPE 202 that is inside the LC cell (the top substrate 101 of the Sato design, shown in FIG. 1, being flipped 180°, upside down). In this case, the HPE 202 and TUE 104 are very close to each other (separated just by the LC 212 of thickness L) and a few volts (<5V) 206 are enough for OP control of the LCL 200. However, left unaddressed the electric field profile would have an abrupt character inside of the LC layer 212. To address abrupt changes in the electric field, a high resistivity or weakly conductive layer (WCL) 214 is cast on the surface of the HPE 202 which smoothens the above mentioned electric field profile due to a very high sheet resistance Rs, which is defined as R=(dσ)−1, where d is the thickness of the WCL 214 and a is its conductivity. This smoothening may be understood by using the concept of attenuation of the electric potential (from the periphery to the center of the HPE 202) in a classical electronic RC circuit where the capacitance of the unit area of overlap between the two electrodes the TUE 104 and the HPE 202 and which is covered by the WCL 214 material containing the dielectric LC layer 212 in between. At the same time, the role of the electrical resistance R is mainly played by the sheet resistance Rs of the WCL 214.
It happens that the “RC factor” of miniature cameras (with a CA 110 in the order of 1.5 mm to 2 mm) and the dielectric properties ∈LC of the LC layer 212 of thickness L are such that the sheet resistance Rs of the WCL 214, that is necessary for a smooth electric field profile, is in the range of tens of MΩ/. The fabrication of such films is a difficult task since manufacturing variations in parameters change noticeably the sheet resistance.
In addition, consumer product cameras are supposed to work with unpolarized light. This requires the use of two LC layers 212 (with their molecular directors being oriented in perpendicular planes, shown in FIG. 3A) to handle two cross oriented polarizations of unpolarized light. To have two such “half” lenses 200 focusing in the same way, two WCLs of the same Rs (within ≈±3%) are needed. This imposes specific conditions on the manufacturing of a polarization independent “full” lens 300. There is another limitation to this approach: the attenuation of the electrical potential (from the periphery of the HPE 202, points A or B, towards the center of the lens, point C) is defined by the physical nature of the RC circuit and thus it is difficult to obtain various aspherical profiles, which are sometimes required to have good optical image quality. Finally, all materials with appropriate Rs values (known so far) are sensitive to temperature variation.
Several alternative approaches have been developed to address, at least in part, the problems of Naumov's geometry. One of them, proposed by LensVector and published in WO2009/153764 which is incorporated herein by reference, is the use of a single WCL 314 to eliminate the severe requirements of manufacturing repeatability, as illustrated in FIG. 4. In this configuration 300, the HPE 302 and the WCL 314 are positioned between two substrates 105 (almost symmetrically) which serve as bottom and top substrates for two cross oriented LC layers 312. Thus, the same control electrode structure (HPE 302 & WCL 314) is used to drive both LC 312 layers similarly.
An alternative approach 400 shown in FIGS. 5A and 5B was proposed by B. Wang, M. Ye, M. Yamaguchi, and S. Sato, “Thin Liquid Crystal Lens with Low Driving Voltages”, Japanese Journal of Applied Physics 48 (2009) 098004. In this geometry, the WCL layer 414 is close to the LC layers 412, while there is an additional electrically connected Disc Shaped Electrode (DSE) 416 in the middle of HPE 402, both being positioned outside of the LC cell 400. While this approach helps to avoid transitory molecular orientational defects (so-called disclinations) and to achieve a better control of the electric field profile 500, it still suffers from several drawbacks, including the problem of manufacturability of the WCL 414. This is because, two WCLs (one for each LC layer) are still needed or only one WCL 414 and control electrode (HPE 402 & DSE 416) may be used but it must be positioned relatively far from the TUE 104 since now two LC layers 412 must be driven by the same HPE 402 & DSE 416 electric field control structure. In addition, there is a need to have two independent continuously variable voltages V1 106 and V2 406 to drive the LCL 400. Thus, the grounding of the TUE 104 and applying the voltage V1 106 on the HPE 402 may create a spatially non-uniform potential distribution and corresponding optical power (as shown schematically 500 by the solid curve in FIG. 5B). At the same time, the application of voltage V2 406 to the DSE 416 may generate a uniform electrical potential (as demonstrated by the solid and dashed horizontal lines, shown in FIG. 5B) avoiding thus the appearance of disclinations or allowing continuous control of the optical power and aberrations of the lens 400. The absolute values of those voltages 106/406 also are still higher than those 206 used in the Naumov's approach (because of the additional distance between electrodes 104/402 imposed by the thickness H of the top electrode 101). This last point was addressed by M. Ye at al. in “Low-Voltage-Driving Liquid Crystal Lens”, Japanese Journal of Applied Physics 49 (2010) 100204, RAPID COMMUNICATION, by flipping the top substrate 101 upside-down, by covering the electrodes with 1 μm SiO2 film and by using (as WCL 414) a highly resistive film of water-borne thermosetting paint (TWH-1, Mitsubishi Materials Electronic Chemicals). Finally, there are still some significant wavefront (aberration) problems with this double voltage 106/406 controlled scheme.
To resolve the remaining aberration (wavefront) problems, LensVector in WO2012/079178, which is incorporated herein by reference, has introduced another approach 600, where a transparent floating (non-connected) conductive layer (generally in the form of a disc) 618 is introduced between the two cross oriented LC layers 212 of a pair of Naumov's design half LCLs 200, used in a full lens geometry 600, shown in FIG. 6. The presence of the floating conductive layer 618 improves significantly (compared to Sato's and Naumov's designs) the wavefront profile and the Modulation Transfer Function (MTF) of cameras using such LC lenses 600. In addition, the unique voltage 206 required for driving the lens 600 is very low and the device 600 operates by frequency control.
Alternative approaches were proposed to resolve all three problems (poor WCL 214 repeatability, high voltage 106/406 and undesired wavefront). One of them 700, proposed by N. Hashimoto, “Liquid Crystal Optical Element and Method for Manufacturing Thereof”, U.S. Pat. No. 7,619,713 B2, Nov. 17, 2009, is shown in FIG. 7. The basic difference of this design, compared to the Naumov's approach, is the absence of the WCL 214. In fact, Hashimoto proposes the use of optically transparent multiple Concentric Ring Shaped Electrodes (CRSE) 718, which are interconnected via high resistivity “bridges” 720 (the schematic side view is shown in FIG. 7 and the top view is shown in FIG. 8). This “resistively-bridged” structure plays the same role as the WCL (214) in creating a voltage spatial profile over the aperture. The advantage of this approach is that the individual resistivity values (R1, R2, etc.) of the bridges 720 can be adjusted to obtain the desired wavefront. Also, two small voltage V1 206 and V2 706 are needed, applied to the center 702 and to the periphery of the external ring shaped electrode 202 with the TUE 104 being grounded to drive the lens 700.
Another approach 800, proposed by Bos et al. “Tunable electro-optic liquid crystal lenses and methods for forming the lenses”, US Patent Application Pub. No.: US 2011/0025955 A1, Feb. 3, 2011, is shown in FIG. 9. In addition to the resistive bridges 720 (described by Hashimoto), describes individually addressable CRSEs 818. However, in addition to manufacturing problems, the dynamic control of such lenses will be complicated (similar to LCDs).
An intermediate solution was proposed by Y. Kato et al. in “Automatic Focusing Apparatus”, US Patent Application Pub. No. US 2007/0268417 A1, Nov. 22, 2007, where a central DSE and a peripheral HPE are each connected to power supplies, with correspondingly voltages V1 and V2, while all intermediate CRSEs are connected via the resistive bridges to the DSE and HPE. This approach also suffers from manufacturability problems.
In the above mentioned proposals, either resistive bridges 720 or individual control of concentric electrodes 718/818 must be controlled and thus wavefront shape control and low voltage may be addressed to a limited degree. However, there are significant drawbacks in those approaches 700/800 also. One of them is the abrupt variation of the electric field, particularly at the periphery of individual electrode segments 718/818. Thus in the area covered by one electrode segment 718/818, the potential is uniform, while there is an abrupt change between those segments 718/818. This requires very close electrode segments 718/818 to minimize the impact of abrupt changes of the electrical potential. In addition, the relatively flat zones in the wavefront will degrade the MTF of the camera and thus a very high number of such electrode segments 718/818 is required. This, in turn increases the requirements on manufacturing precision of those segments 718/818, on the resistive bridges 720 and the dynamic control of voltage distributions of those structures also becomes extremely difficult to handle in practice.